DESCRIPTION

These functions provide the interface for the UNIX file descriptors. File
descriptors can be used to access vnodes (see vnode(9)), sockets (see
socket(2)), pipes (see pipe(2)), kqueues (see kqueue(2)), and various
special purpose communication endpoints.
A new file descriptor is allocated with the function falloc() and freed
with fdrelease(). falloc() and fdrelease() deal with allocating and free-
ing slots in the file descriptor table, expanding the table when neces-
sary and initializing the descriptor. It's possible to do those things in
smaller steps, but it's not recommended to make complicated kernel APIs
that require it.
The files are extracted from the file descriptor table using the func-
tions fd_getfile(), getvnode() and getsock(). fd_getfile() performs all
necessary checks to see if the file descriptor number is within the range
of file descriptor table, and if the descriptor is valid. getsock() and
getvnode() are special cases that besides doing fd_getfile() also check
if the descriptor is a vnode or socket, return the proper errno on error
and increase the use count with FREF().

CONCURRENT ACCESS

Since multiple processes can share the same file descriptor table, it's
important that the file is not freed in one process while some other pro-
cess is still accessing it. To solve that problem a special use count is
kept with the functions FREF() and FRELE(). In most cases FREF() should
be used on a file after it has been extracted from the file descriptor
table and FRELE() should be called when the file won't be used anymore.
There are cases when this isn't necessary, but since FREF() and FRELE()
are cheap to use, there is no reason to risk introducing bugs by not us-
ing them.

SEE ALSO

CODE REFERENCES

The majority of those functions are implemented in
sys/kern/kern_descrip.c. The function prototypes and the macros are lo-
cated in sys/sys/file.h and sys/sys/filedesc.h.
MirOS BSD #10-current August 23, 2002 1